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Investment Securities (Debt securities issued by corporation) (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 2,633.8 $ 2,547.3 $ 2,604.6 $ 1,543.5
Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 718.0 746.3    
AA | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 19.0 37.3    
A | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 170.8 212.8    
BBB | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 300.6 335.6    
BB | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 210.1 143.2    
B | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 17.5 $ 17.4